The present invention relates generally to communications systems and in particular to a technique to perform TDMA timing in a satellite, terrestrial wireless, and cable based systems.
Time Division Multiple Access (TDMA) is one of several techniques used to design distributed satellite, wireless, and cable based systems. A TDMA system provides a single communication channel that is efficiently shared among multiple geographically distributed communication terminals. In such a system, different terminals share a channel""s bandwidth by transmitting on the channel with precisely timed short bursts of data. Because the timing for the bursts must be precise, the TDMA system provides each terminal with a very accurate time base. The time base is used to precisely time the terminal""s transmissions into the channel so that the burst transmissions from different terminals do not overlap in time.
Each of the terminals must acquire the system timing so that the terminal can become synchronized with other terminals within the overall system. However, acquiring a lock to the system timing is difficult and can be lengthy. Furthermore, most systems have a large guard time which allow for uncertainty in timing among the different terminals. A large guard time is undesirable because of the resulting loss of usable bandwidth.
The problem of providing accurate timing is made difficult by a number of factors. First, different stations or terminals have different amounts of propagation delay between terminals. Second, the delay between terminals changes with time as the distance traversed by the transmission terminals changes. For example, in geo-synchronous satellite systems, the delay can be caused by imperfections in satellite orbit. In non-geo-synchronous satellite systems, the delay can be caused by the nature of the satellite orbit. In mobile satellite or terrestrial wireless systems the delay may be caused by the movement of the terminals. Finally, as terminals use a local clock to derive TDMA timing, inaccuracies and variations in the local clock can also cause TDMA timing to drift with the passage of time.
In addition, conventional satellite systems communicate on bursts within the frames which have preallocated parameters such as length, frequencies, and the location of the burst within a frame. These parameters are typically fixed at the terminals, and although they can be changed from time to time, they may not be dynamically allocated or reprogrammed, on the fly, by the network controller. As a result, the network system is rigid and not able to adapt to real-time changes in demand for bandwidth. Therefore, system resources are wasted due to the fixed or pre-established nature of the network architecture.
It is therefore an object of the invention to provide an improved acquisition and synchronization of all terminals in time, so that bursts transmitted by different terminals do not overlap in time, and that a burst transmitted by one terminal arrives at the appropriate time at the receiving terminal.
It is another object of the invention to enable bursts to be positioned close to one another (within a few microseconds) and assure that bursts from different terminals do not overlap in time. As a result, satellite delay variations of several milliseconds should be accommodated according to the invention.
It is yet another object of the invention to allow terminals to adjust their timing in a short period of time after they are turned on.
It is a further object of the invention to allow terminals to join the network without adversely impacting terminals that are actively carrying traffic in the network.
According to an exemplary embodiment of the invention a satellite or wireless based TDMA system uses a programmable, fixed-period frame structure. All bursts are timed with respect to this programmable periodic frame. Each terminal uses its local clock to generate a transmit frame period and a receive frame period. The start of a transmit frame period is known as Start of Transmit Frame. The start of a receive frame period is known as Start of Receive Frame. According to the invention, an Acquisition and Synchronization procedure is provided to align the start of transmit frame and start of receive frame at each terminal in such a way that if a burst is transmitted by any terminal at offset x after a local start of receive frame of the terminal, then all terminals receive that burst at position y after their local SORFs, where |yxe2x88x92x| is less than a small threshold value, for example, 5 microseconds.
In addition, the invention uses a number of bursts for communication between terminals. According to an embodiment of the invention, the various bursts are programmable. In other words, the parameters defining the bursts, such as length, frequency, and location within a frame may be reprogrammed by the network through communication with the terminals. As a result, the parameter may be dynamically allocated based on monitored network conditions. A number of different bursts are used for network communications. Reference Bursts are transmitted by reference terminals and are received by all traffic terminals. The Reference Bursts carry network management messages from the network command controller or reference terminal to all other terminals. Reference Bursts are also used by all receiving terminals to derive frame timing. Signaling bursts are transmitted by traffic terminals and received by the reference terminal. Signaling Bursts are used carry network management messages from the terminals to the network command controller or reference terminal. Traffic bursts are used to carry user traffic. Traffic Bursts are transmitted by traffic terminals and reference terminals and received by the traffic terminals and reference terminals. Acquisition bursts are used during transmit acquisition of traffic terminals. Acquisition Bursts are transmitted by traffic terminals and received by a reference terminal. Control bursts are used to maintain a traffic terminal transmit synchronization. Control Bursts are transmitted by traffic terminals and received by a reference terminal.
According to the present invention, the system is able to handle global, spot, and mixed mode beam configurations. In addition, the system is able to handle multiple spot beams, large numbers of terminals, multiple carriers, and multiple reference bursts. Furthermore, the procedures are simple, uniform, and robust and do not require special purpose hardware support. The system according the exemplary embodiments of the invention can also handle large doppler and local clock variations. As a result, the system is suitable for large delay satellite networks, as well as, low to medium delay terrestrial wireless and cable networks.
The system according to the present invention also provides that all parameters are programmable making it easy to modify and optimize communications for specific networks and real time condition. Procedures are controlled using message exchanges which are not hard assigned to specific frames. This considerably simplifies the implementation of this scheme. Furthermore, the receive acquisition algorithm, according to the various described embodiments of the invention, uses a fixed size aperture which can be stepped in a controlled fashion thereby reducing the probability of false detection of a unique word in the communication burst as the search or track process progresses. In marked contrast, prior techniques depended on pure chance for detection of the right unique word.
According to the exemplary embodiments of the invention, the procedure facilitates a very simple method for reference station switchover. Traffic terminals look at only a reference burst irrespective of which reference station it is transmitted by (if there are multiple reference stations for redundancy), which simplifies the traffic terminal procedure. Receive and Transmit corrections are smooth and orderly both during acquisition and synchronization. The correction information can be used to accurately measure the round trip time to the satellite, the Doppler, and clock inaccuracy. Terminals can be acquired in a live network, without disruption of existing traffic and without human intervention. The system also requires very small bandwidth overhead. Additionally, a fail safe procedure is provided whereby terminals automatically stop transmitting if sync is lost.